This application bases its priority on provisional application Ser. No. 60/027,920 filed on Oct. 11, 1996.
The present invention relates to immersion heaters for heating a liquid in a bath. More particularly, the invention relates to a grounded gas purged immersion heater.
Tubular electric heating elements are known in the art to consist of a resistance wire coil or ribbon wound in such a way as to provide an exact electrical resistance for a given length of coil. The coil is generally inserted into a sheath, usually a tube made of metal, and filled with an electrically insulating material, such as magnesium oxide. The assembly is then roll reduced or swaged to compact the fill material and eliminate any voids within the assembly so as to facilitate heat transfer. The entire structure is then annealed to eliminate stresses built up during roll reduction. The finished heating element can then be formed into an unlimited variety of shapes or configurations as needed for the process requiring heat.
It is also known in the art that watt densities or the amount of heat which can be transferred from a given length of tubular heating element varies depending upon the process for which the heater element is used. As an example, an oil based liquid transfers heat much more slowly than does a water based liquid. Since the resistance wire must stay well below its melting point to provide economical, useful life, the amount of power (or watts) for a unit area must be varied. A common "watt density" known in the art for heating an oil type liquid is 20 watts per square inch of heater sheath area. For a water based liquid, watt densities can be as high as 90 watts per square inch.
From the above, it is evident that for any given application, a certain amount of material must be used to achieve the proper watt density. Therefore, it would be beneficial if one could use less material to provide an equivalent amount of surface area. If this were done, a cost saving would be achieved.
Many shapes have been used for tubular heater sheaths. It is common in the art to use triangular, oval or even flat surfaces on the sheaths in order to increase heater efficiencies. Protrusions along the heater sheath, such as fins, splines or pins, have also been used and work very well for certain applications. Each of the shapes described, however, has specific limitations. Flat and oval sheaths lack the ability to maintain sufficient compacting of the fill material. This in turn can produce voids within the heater element thus limiting heat transfer. Fins and other protrusions increase the amount of surface area but also require additional manufacturing steps, as well as additional material. Both of these increase costs. It would be desirable to increase the surface area of a tubular heating element without adding material or requiring additional shaping.
Electrical resistance heaters formed of a continuous flexible cable are particularly suitable for immersion in corrosive chemical baths since the exterior of the flexible cable may be jacketed with a suitable plastic material having satisfactory resistance to the corrosive nature of the chemical bath being heated. An example of a flexible cable resistance heater is shown and described in U.S. Pat. No. 4,158,764. This patent is incorporated herein by reference in its entirety.
It is known to provide such flexible cable heaters with an outer casing or jacket formed of a polytetrafluoroethylene (PTFE) material. PTFE has satisfactory resistance to chemical attack by corrosive media. However, it has the disadvantage that when employed in a thin walled tube for desired flexibility, the permeability of PTFE permits transmigration of heated chemical vapor into the interior of the cable heater. To overcome this problem, U.S. Pat. No. 4,553,024 discloses that the outer jacket of the cable-type immersion heater can be connected to a suitable source of a dry gaseous medium for circulation from an inlet end of the heater cable through the interior thereof, and over the heating element, to an exhaust at the other end of the heater cable. This provides a continuous dry gas flow or purge over the resistance heating element to scavenge any accumulated corrosive chemical vapors which may have permeated through the outer plastic jacket of the heater cable. Pat. No. 4,553,024 is also incorporated herein by reference in its entirety.
One of the difficulties with the flexible cable heaters illustrated in U.S. Pat. Nos. 4,158,764 and 4,553,024 is that the heaters are not grounded. Such grounding is required by various regulatory authorities, such as Underwriters Laboratories (UL) and the Canadian Standards Association (CSA) in order to be approved. It would also be desirable to have a gas purge take place on such grounded flexible cable heaters while maintaining good heat transfer through the PTFE jacket of the cable heater.
Accordingly, it has been considered desirable to develop a new and improved heater sheath element which can be used in a purged grounded fluid heater to meet the above-stated needs and overcome the foregoing difficulties and others while providing better and more advantageous overall results.